This invention relates to flexible collapsible plastic containers, also referred to herein as bags, for medical solutions such as are used in intravenous fluid therapy, and more specifically to such bags which are calibrated for determining the volume or rate of administering medical solutions.
The flexible plastic containers have many advantages over glass containers including reduced weight, lack of susceptibility to breakage, reduced storage space requirements, and the potential for gravity drainage of liquid from the bag without the need to replace the liquid removed from the bag with air.
The accurate measurement of the volume of liquid in a bag is very important in many instances. For example, it may be important in determining the volume of liquid administered, and thereby monitoring the rate of infusion of liquid, and in verifying infusion pump volume indications. As a result of these needs, bags provided with calibrations are available. However, there has been concern about the accuracy of the measurements of the volume of liquid within flexible plastic containers based on these calibrations. One of the difficulties of accurately measuring the volume of liquid occurs when no headspace air is present, because as the liquid is withdrawn, the bag simply collapses, the two opposing surfaces of that portion of the bag gradually being forced together. As will be described in more detail below, there is no well-defined air-liquid interface or meniscus, and consequently there is the high probability of making a significant error in determining the volume of liquid which remains in the bag on the basis of calibrations thereon. Currently, a reading based on the calibration of containers is dependent upon a discernible air-liquid interface, but such a reading will vary considerably with the volume of headspace air. For example, 3000 mL of fluid in a flexible film container measured 3200 mL with 5 mL of headspace air, but only 2300 mL with 150 mL of headspace air.